What's really in your swimsuit, what happens to it in the heat, and what to wear instead

I love a sauna. The heat that sinks right into your bones. The quiet. That lovely heavy feeling afterwards when your whole body has finally let go.

But a few years ago I started asking a question I couldn't shake. Why is almost all swimwear made from plastic?

Once you ask that, a second question follows pretty quickly. What happens to that plastic when you sit in a room heated to somewhere between 70 and 100°C, sweating, for 15 minutes?

It's not about fear, it's about awareness.

This week we celebrated the launch of Earthcove in a London sauna, surrounded by friends talking about exactly this. So let's look at what's really going on.

What is most swimwear made from?

Pick up almost any swimsuit and check the label. You'll usually find polyester or nylon (also called polyamide) blended with elastane (also sold as spandex or Lycra).

A typical swimwear fabric is around 80% polyester or nylon and 20% elastane. All three are synthetic fibres made from fossil fuels. In other words, plastic.

They're used because they stretch, hold their shape and dry fast. But synthetic fabrics are often treated with chemical finishes to get there. That's where things get more concerning.

What chemicals are in plastic swimwear?

Not every swimsuit contains every chemical below. That matters. But these three come up again and again in independent testing of synthetic clothing, and each one has a reason for being there.

What is PFAS, and why is it in swimwear?

PFAS (per and polyfluoroalkyl substances) are a family of more than 10,000 synthetic chemicals. They're known as "forever chemicals" because they barely break down in the environment or in our bodies.

In clothing, PFAS are used to make fabric repel water, stains and grease. In swimwear, that means water resistance and quick drying.

And they do turn up in swimwear. When the Environmental Working Group tested children's products, they found PFAS in kids' swimwear from Carter's and Disney. A global study by IPEN and partner organisations tested 72 clothing items, including swimsuits, and found PFAS in 64% of them.

What does PFAS do to our health?

PFAS are known to cause harm including a lowered immune response to vaccination, impaired liver function and lower birth weight. Some build up in the body over time. Children are especially vulnerable because of their smaller bodies and developing organs.

What are phthalates, and why are they in swimwear?

Phthalates are chemicals used to make hard plastics soft and flexible. They're mostly used to soften PVC.

You might assume they're what turns plastic into a stretchy fabric. That's not quite how it works. Polyester and nylon fibres aren't usually softened with phthalates. In clothing, phthalates mainly turn up in plastisol prints (those thick, rubbery printed logos and patterns), PVC coatings, synthetic leather trims and some printing inks.

So a swimsuit with a big rubbery print or a plastic coated trim is more likely to be a source. Greenpeace testing has found phthalates in the printed areas of everyday clothing at up to 11% by weight.

What do phthalates do to our health?

Phthalates are hormone disruptors. Studies have linked phthalate exposure with a range of health problems and shown effects that mimic oestrogen in the lab. That's why several phthalates are restricted in children's toys and childcare items in the EU and US.

What is BPA, and why is it in swimwear?

BPA (bisphenol A) is a chemical used to make plastics clear and hard. Most of us know it from the push to get it out of baby bottles.

Fewer people know it turns up in clothing. According to the Center for Environmental Health (CEH), BPA can be added during polyester manufacturing to improve the fabric's properties and lifespan. Other reports suggest links to dye fixing and antistatic finishes, although that's less well documented.

CEH has tested sports bras, leggings and athletic shirts made from polyester and spandex. That's the same fibre family used in most swimwear. Some items could expose wearers to up to 40 times California's safe limit for BPA. CEH has not found BPA in socks made mainly from cotton or other natural fibres.

What does BPA do to our health?

BPA mimics oestrogen. It can disrupt metabolism, growth, development and reproduction. And it can be absorbed through skin. Handling a till receipt for a few minutes is enough for BPA to reach the bloodstream.

The plastics industry maintains that BPA is safe at the low levels most people are exposed to. Many scientists disagree, and research is ongoing.

Why you shouldn't sweat in plastic swimwear in the sauna

Here's where the sauna changes things.

You'll often hear that heat "opens your pores". The science is a little different, but it lands in a similar place. Heat increases blood flow to the skin. Sweat keeps skin warm and wet. Both make skin more permeable.

In one study using human skin samples, raising skin temperature from 25°C to 39°C increased absorption of every substance tested. A US Army study found the same pattern with clothing. Heat and humidity can increase how much chemical leaches out of fabric, and sweat and skin hydration can increase how much gets absorbed.

We also know PFAS can cross the skin. A University of Birmingham study found 15 of 17 PFAS tested were absorbed through a model of human skin in substantial amounts.

In a sauna, your skin gets hotter than 39°C. You're sitting still and sweating, often for 15 minutes or more. And your swimsuit is pressed tight against your skin the whole time.

Is it harmful to sweat in plastics?

I want to be upfront here. Nobody has yet run a study measuring exactly what happens to swimwear chemicals in a sauna.

But the building blocks are clear. Synthetic fabrics can carry PFAS, BPA and phthalates. These chemicals can be absorbed through skin. Heat and sweat increase absorption. Put those together and the sauna is about the worst place to wear a plastic swimsuit.

It's not only your skin. It's the air too.

There's another route that doesn't get talked about much. Synthetic fabrics release volatile chemicals into the air, and heat speeds this up. One study on synthetic textiles found emissions of hazardous volatile compounds rose significantly when the temperature went from 25°C to 50°C.

A sauna is far hotter than that. It's also a small, enclosed room shared with other people. So a sauna full of plastic swimwear isn't only about what touches your skin. It's about what everyone in there is breathing. Research on this is still early. But it's one more reason to think about what we wear into the heat.

Why do people sauna? The lovely benefits

Let's balance this out, because saunas are wonderful.

The word sauna comes from Finland, where there are around 3 million saunas for 5.5 million people. Finnish sauna culture is recognised by UNESCO as part of the world's intangible cultural heritage. For Finns, it's about cleansing the body and the mind.

The research is encouraging too. A Finnish study followed more than 2,000 men for around 20 years. Those who used a sauna four to seven times a week had a 40% lower risk of death from any cause than those who went once a week. It's an observational study, so it shows a link rather than proof. But it's a lovely link.

People sauna to relax, to recover after exercise, to warm up in winter and to spend quiet time with friends. And, for many of us, to sweat things out.

Sauna detox: sweating it out, or sweating it back in?

One of the big reasons people sauna is to sweat out toxins. There's some interesting evidence here. Small studies from the University of Alberta found BPA in the sweat of 16 out of 20 people, including some with no BPA detected in their blood or urine. Phthalates showed up in sweat too. The researchers concluded that sweating may help the body eliminate these chemicals.

These were small studies, and your liver and kidneys do most of the heavy lifting. But if sweating is part of why you sauna, it makes no sense to sit there wrapped in fabric that may carry the very same chemicals.

That's not detoxing. That's retoxing.

What is the best thing to wear in a sauna?

Saunas were made for bare skin. In the Finnish tradition you go in naked and sit on a cotton or linen towel. Nothing between you and the heat. It's still the most natural way to sauna.

But most public saunas in the UK, the US, France and many other countries require swimwear. So what's the next best thing?

What makes swimwear sauna-safe?

• Natural fibres first. Look for swimwear made mostly from organic cotton or other plant fibres, rather than polyester or nylon.

• Independent certification. A label that says "eco" or "natural" isn't enough. GOTS (the Global Organic Textile Standard) is the leading standard for organic textiles. It covers the whole supply chain, from the organic fibre to the finished garment, and sets strict rules on the chemicals used along the way.

• No rubbery prints or plastic coatings. These are where phthalates tend to hide.

• No metal. Clasps, underwires and rings heat up fast and can burn your skin.

• A loose, breathable fit. Tight synthetics trap heat and sweat against your skin.

Practical tips for your next sauna

• Sit on a cotton towel. It's better for hygiene and puts a natural layer between you and the bench.

• Rinse off first. Try not to go in wearing a swimsuit still soaked in pool chlorine.

• Go bare where you can. If you have access to a private sauna, do it the Finnish way.

• Drink plenty of water. Before and after, every time.

• Start with one swap. Replace the swimsuit you wear most. You don't need to replace everything at once.

Sauna-safe swimwear from Earthcove

This is exactly why we created Earthcove.

Earthcove is the world's first organic cotton swimwear range. Every piece is GOTS certified and UPF 50+. It's made from 98% organic cotton, with 2% Lycra for stretch and fit. We're open about that 2% because we'd always rather tell you the full story than make a claim we can't stand behind.

Earthcove swimwear is made without PFAS, phthalates, formaldehyde, BPA or azo dyes. Made from plants, not plastics. For kids and grown ups. For the pool, the sea and the sauna.

If you're looking for sauna-safe swimwear, have a look at the Earthcove range.

Every small change adds up

You don't need to overhaul your entire life overnight. Maybe it's one swimsuit. Maybe it's a cotton towel for the sauna bench. Maybe it's going bare next time you're somewhere that allows it.

Every small change adds up. And remember, it's not about fear, it's about awareness.

Enjoy the heat. And let it do its good work.

Emma x

Sauna-safe swimwear: your questions answered

What should I wear in a sauna?

Traditionally, nothing at all, sitting on a cotton towel. In public saunas where swimwear is required, choose loose, breathable swimwear made from certified organic natural fibres, with no metal parts or rubbery prints.

Is it safe to wear a polyester swimsuit in a sauna?

Polyester won't melt at normal sauna temperatures, but it isn't the best choice. Polyester and nylon swimwear can carry chemical finishes and residues such as PFAS and BPA. Heat and sweat increase how much of a chemical your skin absorbs.

Can you wear a cotton swimsuit in a sauna?

Yes. Cotton is breathable, copes well with heat and doesn't need the synthetic finishes common in plastic swimwear. Look for GOTS certified organic cotton for independent assurance on the chemicals used.

What is the best fabric for sauna swimwear?

Natural fibres from plants, ideally certified organic cotton. GOTS certification covers the fibre, the processing and the finished garment, including strict rules on harmful chemicals.

Does sweating in a sauna help you detox?

Small studies have found chemicals such as BPA and phthalates in sweat, which suggests sweating may play a role in clearing them. Your liver and kidneys do most of the work. Either way, it makes sense not to sweat in fabric that may carry those same chemicals.

Sources

1. The Conversation (2026) Swimwear: to rinse or not to rinse?, republished by CityNews. citynews.com.au/2026/swimwear-to-rinse-or-not-to-rinse (secondary source)

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